Glazing panels fire resistant in accordance with the G fire resistance classes, together with their frames and their fittings, must offer resistance, in a fire withstand test according to the standard DIN 4102 or to the standard ISO/DIS 834-1, for a certain time, to the passage of the fire and smoke. During this time, the glazing panels must neither break, under the effect of the stresses which occur as a result of the temperature gradients between the surface of the glazing panel in contact with the heat and the embedded edge, nor exceed their softening point, since they would lose their stability and would thus expose the opening. They are ranked in the fire resistance classes G 30, G 60, G 90 or G 120 depending on the time in minutes for which they withstand fire.
In general, fire-resistant glazing panels are held in frames which protect, to a greater or lesser extent, the edge of the said glazing panels from the effect of the heat. The temperature gradient which thus occurs between the middle of the glazing panel and the edge generates considerable tensile stresses in the marginal region and results in the destruction of the glazing panels if special measures are not taken to compensate for these tensile stresses. These measures consist of thermally toughening the glazing panels, this toughening making it possible to induce large initial compressive stresses in the marginal region. The thermal toughening gives the glazing panel additional properties of a safety glass when the toughening is carried out in such a way that, should the glazing panel break, it would do so by fragmenting into tiny pieces.
The initial stress state is usually determined by means of the flexural/tensile strength obtained by the toughening operation, in accordance with the standard DIN 52303 or to the standard EN 12150. Experiments have in this case shown the need to guarantee a flexural/tensile strength of at least 120 N/mm2 so that the glazing panel can withstand the tensile stresses generated by the temperature gradients at the edge. Given that untoughened glazing panels have a basic flexural/tensile strength of approximately 50 N/mm2, this means that it is necessary to increase this strength, by toughening, by at least 70 N/mm2. The value of this increase in the flexural/tensile strength corresponds directly to the value of the initial compressive surface stresses.
It is also possible to increase the fire resistance time by increasing the depth of insertion of the glazing panel in the frame. In the case of a flexural/tensile strength of the glazing panel of 120 N/mm2 and an insertion depth of 10 mm, the glazing panel conforms, for example, to the fire resistance class G 30, while an insertion depth of 20 mm allows it to achieve the fire resistance class G 90.
Glazing panels made of the usual float glass (soda-lime-based silica glass) may be suitably toughened by means of conventional toughening plants, given that these glass compositions have relatively high thermal expansion coefficients, greater than 85×10−7 K−1. The usual float glass allows flexural/tensile strengths possibly ranging up to 200 N/mm2 to be achieved. Under the effect of the tensile stresses generated by the temperature gradients, the glazing panels consequently do not break if the insertion depth is approximately 10 mm, but they lose their stability because of their relatively low softening temperature of approximately 730° C. Toughened glazing panels made of float glass therefore conform, under standard installation conditions, at the very most to the fire resistance class G 30.
However, monolithic glazing panels of fire resistance class G 60 and higher classes are also known. These glazing panels consist of glass compositions having a high softening point of greater than 815° C. and consequently have a long resistance time in a fire withstand test. In this case, borosilicate- and aluminosilicate-based heat-resistant glasses prove to be particularly suitable. However, these types of glass must also be toughened thermally in order to be able to withstand the high tensile stresses which occur in the marginal region in a fire withstand test.
The use of thermal toughening for fireproof glazing panels whose glass compositions are based on heat-resistant borosilicate or on heat-resistant aluminosilicate is known from the documents DE 2,313,442 B2 and U.S. Pat. No. 3,984,252. According to these documents, only suitable for toughening are glasses for which the product of the thermal expansion α and the modulus of elasticity E reaches 1 to 5 kg·cm−2·° C., i.e. borosilicate- or aluminosilicate-based glasses having a thermal expansion of α20-300=30 to 65×10−7 ° C.−1. However, the necessary toughening at the edge of these glazing panels cannot be carried out by means of conventional air-toughening plants but requires a special process in which the glazing panels are placed, during the heating, between slightly smaller ceramic tiles in such a way that the edge of the glazing panel extends beyond the ceramic tiles and is therefore cooled more rapidly, while the middle of the glazing panel cools more slowly due to the effect of the ceramic tiles. The necessary toughening at the edge may, to be sure, be achieved in this way, but the glazing panels thus manufactured do not have any safety-glass properties.
It is known from the document EP-A-638,526 to use, for the manufacture of monolithic fireproof glazing panels, glass compositions which have a thermal expansion coefficient α of between 30 and 60×10−7 K−1, a φ coefficient of between 0.3 and 0.5 N/(mm2·K), a softening point (=temperature for a viscosity of 107.6 poise) of greater than 830° C. and a working point (=temperature for a viscosity of 104 poise) of between 1190° and 1260° C. The φ coefficient or specific thermal stress is the specific parameter of the glass calculated from the thermal expansion coefficient α, the modulus of elasticity E and Poisson's ratio μ according to the formula φ=α.E/(1-μ). Glazing panels having these physical properties may acquire, in a conventional air-toughening plant, both the initial compressive stresses necessary at the edge and the toughening stresses exerted over the entire surface and necessary for obtaining fragmentation into tiny pieces, so that no particular measurement is necessary in respect of the toughening operation and so that the manufacturing process is thereby considerably simplified. However, glazing panels having these physical properties necessarily contain B2O3, Al2O3 and ZrO2 in quantities which complicate the melting process and the conversion process. These glazing panels thus cannot be manufactured using the floating process which has proved to be exceptionally economical, given that their conversion point is too high and that the melting furthermore requires special measures.
Borosilicate-based glass compositions are known, from the document FR-2,389,582, which are provided, to be sure, for use in fireproof glazing panels which, because of their relatively low conversion point, may melt during the floating process and also be toughened by means of conventional toughening plants. However, these glasses contain from 11.5 to 14.5% of B2O3 and also have physical properties similar to those of the glasses known from the document EP-A-638,526. Even in the case of these glasses, the initial compressive stresses and the flexural or the tensile strength which may be achieved by air toughening are limited to relatively low values and these glasses also have the known difficulties and drawbacks when melting borosilicate-based glasses.
With regard to the manufacture of emissive screens of the plasma-screen type, the substrate is subjected to several heat treatments for the purpose of stabilizing the dimensions of the said substrate and of fixing a series of layers of various compounds, such as enamels, deposited on its surface. Fixing these relatively thick layers requires the substrate to be heated to temperatures greater than 550° C. If the expansion coefficient of the silica-soda-lime glass used is of the same order of magnitude as that of the compounds deposited on its surface, its temperature withstand is insufficient and it is necessary to place it on a ground slab during the heat treatments in order to avoid any deformation.
Novel families of glass compositions have been developed and described in the patent WO-96/11887 so as to mitigate these drawbacks, especially so as to be able to manufacture sheets or substrates undergoing virtually zero deformation during heat treatments of the order of 550 to 600° C. and capable of generating, by thermal toughening, stress levels comparable to those obtained with standard silica-soda-lime glass.
However, it appears that these glasses may undergo breaks during the deposition of certain layers, including when the methods of depositing these layers result in local temperatures of the glass which do not exceed about a hundred degrees Celsius.
The inventors have thus sought to remedy these breaks, which, albeit infrequent, disrupt the manufacturing plants.